1. Technical Field
The technical field relates to an electronic device that can communicate with an external device, particularly to an electronic device that can transmit a control signal to an external device to change a power state of the external device. The technical field also relates to a power control method between the electronic device and the external device.
2. Related Art
JP-9-51584A discloses an electronic device that can transmit a control signal to an external device to control operation of the external device. In JP-9-51584A, a VTR (video tape recorder) is connected to a TV (television set) through a control line to supply a control signal to the TV through the control line. When receiving the control signal, the TV operates according to the control signal. HDMI (High Definition Multimedia Interface) can be used, for example, as means for connecting the TV and VTR. HDMI cable according to HDMI includes a signal line and a control line.
An example of control of the video camera through HDMI will be described below. In the following description, it is assumed that power state of the video camera is “ON” and the video camera is connected to the TV through the HDMI cable.
When connected to the TV through the HDMI cable, the video camera detects that communication with the TV based on the control signal is turned to be enable from disable. Then the video camera obtains information on the power state of the TV. When the power state of TV is “standby” state, the video camera transmits a power control signal to the TV to switch the power state of the TV from “standby” to “ON”.
However, according to the aforementioned control, when the power state of the TV is changed from “standby” state to “OFF” state due to an influence of an power outage and is subsequently changed from “OFF” state to “standby” state, the following problem occurs in the control.
The video camera usually operates based on an electric power supplied from a secondary battery. Therefore, even in the power outage, the power state of the video camera is not changed, that is, remains in “ON”. On the other hand, the TV usually operates based on electric power supplied from commercial power source. Therefore, when the power outage occurs while the power state of the TV is “standby” state, the supply of electric power is stopped to turn the power state of the TV to “OFF”. Thereafter, when the power outage is eliminated, the electric power is supplied to the TV again, causing the power state of the TV to become “standby” (note: the power state of the TV may not become “standby” depending on the specification).
For this reason, the video camera with power state of “ON” at the power outage detects that the communication based on the control signal becomes “disable” by the power state of the TV changing from “ON” or “standby” to “OFF”. Thereafter, when the power outage stops, the video camera detects that the communication based on the control signal is switched from “disable” state to “enable” state by changing the power state of the TV from “OFF” to “standby”. Then, as described above, the video camera obtains the power state of the TV. Since the obtained power state of the TV is “standby”, the video camera transmits the power control signal in order to change the power state of the TV to “ON” so that TV is turned on.
When the power state of the TV is changed by occurrence/stop of the power outage, the video camera controls the power supply of the TV to change the power state from “standby” to “ON”. In this manner, when the power outage occurs while the TV connected to the video camera is in “standby” state and then the power outage stops, the power state of the TV is automatically changed from “standby” to “ON”, and then the power supply of the TV remains in “ON” state. That is, although the user has intended to set the power state of the TV to “standby”, the TV is turned on without user's knowing.